Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling operations and diurnal engine operation, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. Evaporative leaks may be identified by sealing off the emission control system after generating a vacuum therein, and then monitoring the vacuum bleed-up. In hybrid vehicles, leak detection routines may be adapted to account for the reduced engine operation times which can lead to insufficient vacuum for leak detection and purging routines.
One example approach for detecting evaporative leaks in a hybrid vehicle is shown by Chung et al. in U.S. Pat. No. 7,562,559. Therein, when the vehicle engine is not running, or during an electric drive mode, fuel injection and throttle operation to the engine is disabled while an electric motor or generator is operated to spin the engine and generate vacuum for a leak detection operation. After generating sufficient vacuum, the emission control system is sealed and evaporative leaks are diagnosed.
However, the inventors herein have identified potential issues with such an approach. As one example, when the engine is being spun to generate vacuum, fuel vapors may be drawn from the fuel tank into the engine intake manifold. The drawn fuel vapors may lead to air-to-fuel ratio variations during subsequent engine operation. The situation may be exacerbated if the engine is spun during hot ambient conditions that lead to the generation of more diurnal fuel vapors. As another example, if the engine is spun to generate vacuum while the vehicle is stationary during fuel tank refueling, or while the vehicle operator is inspecting the vehicle, the operator may be startled.
Thus, in one example, some of the above issues may be at least partly addressed by a method of operating an emission control system comprising, during a first key-on condition, spinning the engine fueled and storing fuel tank vapors in a first, larger canister, and during a second key-off condition, spinning the engine unfueled and storing fuel tank vapors in a second, smaller canister. In this way, fuel vapors drawn from the fuel tank during vacuum generation may be stored in an auxiliary in-line canister that is distinct from the main fuel vapor canister.
For example, during specified conditions, such as when the engine has not been running for a threshold amount of time, the engine may be spun with assistance from a starter motor, without any air or fuel injection into the engine cylinders, to generate vacuum for a subsequent leak detection routine. Following vacuum generation, the vacuum may be applied to the engine's fuel system to identify fuel system leaks (e.g., from the fuel tank and/or from the main, larger fuel vapor canister). Vacuum generation may be disabled if the fuel tank is being refueled, if the ambient temperature is higher than a threshold, and/or if the vehicle operator is in the vehicle. Fuel vapors drawn from the fuel tank during vacuum generation may be stored in the auxiliary smaller canister coupled in-line between the fuel tank and the intake manifold. Purging of fuel vapors stored in the auxiliary canister may be coordinated with the purging of fuel vapors from the main canister during subsequent engine operation.
In this way, an engine may be spun without fuel injection to generate a vacuum for evaporative leak diagnostics, while fuel tank vapors are isolated from the spinning engine by an auxiliary canister. By reducing the amount of fuel vapors drawn into the intake manifold during vacuum generation, air-to-fuel ratio deviations during a subsequent engine operation may be reduced. Also, by disabling the vacuum generation under specified engine-off conditions, the vehicle operator may not be alarmed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.